The present disclosure is directed to a mobile machine, such as hydronic surface heater designed to be transported to a potentially remote worksite and operated for an extended period of time. The machine has a fueled component, such as a burner, fueled by a fuel supply system having at least two fuel tanks that are operably connected to one another by a connection line having an electronically controlled valve therein. The valve is coupled to the machine's electrical system, such as being coupled to the output of the machine's main breaker, so as to be opened whenever the machine is operating but to be otherwise closed. The fuel supply system thus has, in effect, a single tank when the machine is running and multiple separated tanks when the machine is not running. Fuel spill risks therefore are mitigated without having to sacrifice operating time and without significantly complicating the machine's fuel supply systems or its controls.